Optical power of a lens

The optical power of a lens is the reciprocal of its focal length:

$D=\frac{1}{f}$

When $f$ is given in meters, $D$ comes out in diopters (1 D = 1 m⁻¹). This is exactly the quantity written on eyeglass prescriptions — a converging lens has $D>0$, a diverging one $D<0$.

For a system of thin lenses in contact with one another, the optical powers add up:

$D=D_1+D_2$

so a converging and a diverging lens can partly cancel each other out. A lens with a focal length of 20 cm has a power of +5 diopters.